This invention relates to memory devices including the discs and disc packs.
A flat and circular disc coated with a magnetic recording material is a common type of memory device in computers. A single disc attached to a hub can be rotatably mounted on a spindle through the hub, or a plurality of discs can be mounted on the spindle as a disc module or pack. In either case, a magnetic head is supported near the disc and is movable radially thereof. Radial translation of the head, together with rotation of the disc, permits selective positioning of the head on the disc surface for reading or recording data.
In the case of a single disc mounted on a hub, it is often desirable to have interchangeability of single disc-hub assemblies on the same drive spindle. To insure convenience of changiang discs, an annular permanent magnetic mounted on the spindle is used to attract a correspondingly shaped and sized armature plate mounted to the aluminum hub carrying the disc. Attraction between the magnet and armature plate maintains the hub against the spindle. In order to center the hub on the spindle, a female truncated cone is formed in the prior art hub and adapted to center itself on a corresponding male truncated cone at the center of the spindle. A diaphragm between the hub cone and periphery is sufficiently bendable so that ideally, after the two cones are completely engaged, some continued flexure takes place permitting joining of the hub periphery and spindle periphery.
Due to the minute spacing between adjacent data tracks on the disc recording surface, extremely accurate initial centering and repeatability of centering are vital to proper recording and reading on the disc. Accuracy is impaired as the magnetic force can cause the disc to be pulled to the spindle at the periphery before proper centering. This problem is recognized in U.S. Pat. No. 3,706,085 to Mowrey granted Dec. 12, 1972 in which the proffered solution is the replacement of the spindle permanent magnet with an electromagnet to be energized after the centers are fit together. The premature off-center joinder can cause dents and nicks in the aluminum cast female cone of the hub due to sharp edges of the male cone and also from any foreign particles trapped between the two cones. Moreover, as damage could result from even a minor mismatch in size, extremely accurate machining of the two cones is required.
A further impediment to accurate centering results from the manner in which the aluminum hub is manufactured. Typically, this hub is die cast and then allowed to cool in ambient surroundings. The outer collar of the hub, being significantly more massive than the diaphragm, takes more time to cool. As some shrinkage accompanies cooling, the continued shrinkage of the outer collar after the diaphragm has cooled can induce compressive and tensile stresses into the diaphragm. As force in the diaphragm must be dependent only upon diaphragm deflection for optimum centering accuracy, these residual stresses, uncettain in direction, can interfere with the elastic bending force.
In the case of a disc module mounted to the spindle, the problems inherent with magnetic attraction can be avoided if the module is mounted to the spindle at the manufacturing site, prior to the recording of servo tracks. If a problem develops in the field, however, the spindle and pack must be returned from the user as a unit for rework or replacement, at significant expense and lost time.